Start `linear_function` canonicalization skeleton copying from
`differentiable_function` canonicalization. For now, transpose function
operands are filled in with `undef`.
Clarify `llvm_unreachable` after exhaustive switch is needed to silence
MSVC C4715, so we don't accidentally remove it.
Standardize some assertion messages.
Move differentiation-related SILOptimizer files to
{include/swift,lib}/SILOptimizer/Differentiation/.
This reduces directory nesting and gathers files together.
